Thin films on semiconductor substrates often cause curvature due to residual stresses. For example, a metal layer deposited on a silicon membrane will cause warping of the membrane. When a microelectromechanical system (MEMS) scanning mirror is formed on the membrane, such warping may introduce optical aberrations because the warping may cause the surface of the mirror to deviate from an ideally flat mirror surface.
Typically, the substrate layer that the thin film is deposited onto is designed to be sufficiently thick to withstand the residual stress with minimal deviations that may occur due to warping. For structures designed to have movable parts, such as scanning mirrors, this structural layer thickness makes it difficult to design members that have low enough stiffness to enable a reasonable amount of motion under an available actuation force. MEMS scanning mirrors, for example, should have a suspension that is flexible enough to allow the scanning mirror to deflect by the desired amount within actuator limits. As a result, there may be competing requirements for the structural layer: sufficient thickness to prevent warping on the one hand, and sufficient thinness to avoid resisting the actuation forces on the other.
Adding thickness to the moving parts of a MEMS scanning mirror may be detrimental because there are likely to be some minimal acceptable resonant frequencies for some natural modes of resonance. Extra thickness of the moving parts adds to the effective mass of the natural modes, so such higher effective stiffness will be required to satisfy minimum resonant frequency requirements. Higher stiffness tends to increase the amount of actuation force needed to achieve a desired deflection of the moving parts, thereby increasing the power requirements to operate the device. A higher stiffness requirement also tends to increase the size of a MEMS device because a stiffer suspension generally has a larger size to keep stress levels within safe limits.
It will be appreciated that for simplicity and/or clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, if considered appropriate, reference numerals have been repeated among the figures to indicate corresponding and/or analogous elements.